It has been discovered that the activity of a particular enzyme, polyisoprenylated methylated protein methyl esterase (PMPMEase which is also known as human carboxylesterase 1 or hCE1), is elevated in certain cancers, particularly in breast cancer. The present invention is methods for the diagnosis and treatment of cancer, particularly breast cancer, by measuring the activity of the enzyme PMPMEase.
Breast cancer is not one disease but several different diseases. Three types of breast cancer are estrogen receptor (ER)-positive breast cancer, progesterone receptor (PR)-positive breast cancer (together termed hormone receptor positive breast cancer), and human epidermal growth factor receptor 2 (HER2)-positive breast cancer (triple positive breast cancer refers to the presence of ER, PR, and HER2 receptors). These subtypes of breast cancer are generally diagnosed based upon the presence, or lack of, these three receptors. In addition, the most successful treatments for breast cancer target these receptors. Another type of breast cancer is called triple negative breast cancer, since none of these three receptors are found in the offending tumor. Because of its triple negative status, triple negative tumors generally do not respond to receptor targeted treatments. Depending on the stage of its diagnosis, triple negative breast cancer can be particularly aggressive, and more likely to recur than other subtypes of breast cancer. Although it cannot be treated with receptor targeted treatments it is commonly receptive to chemotherapy. The lack of targets for treating triple negative breast cancer implies that more research needs to be pursued to find what drives this form of cancer as well as other cancers of unknown etiologies.
Proper diagnosis of the type of breast cancer is essential for proper treatment.
Protein polyisoprenylation and subsequent methylation are essential modifications on a significant proportion of eukaryotic proteins. The modifications are a series of post-translational modifications involving motifs such as -CAAX wherein C is cysteine, A is any aliphatic amino acid, and X is any amino acid whose nature specifies either farnesylation or geranylgeranylation. The modifications include polyisoprenylation of the cysteine of the -CAAX motif (on the sulfur), proteolysis of the carboxyl-terminal three amino acids (AAX), and methylation of the carboxyl group of the cysteine. In the polyisoprenylation step, a 15 carbon (trans, trans-farnesyl) or 20 carbon (all trans-geranylgeranyl)hydrocarbon group is covalently added to the protein.
The only reversible step in the process is the last step, methylation. Two enzymes mediate this final state of polyisoprenylated proteins. Polyisoprenylated protein methyl transferase (PPMTase), also known as isoprenyl carboxylmethyl transferase (ICMT), transfers a methyl group from S-adenosyl-L-methionine (SAM) to the C-terminal —COO− to form the methylated polyisoprenylated protein. PPMTase is essential to the developing embryo; knockout mice lacking PPMTase activity do not survive through mid-gestation. The second of the two enzymes is polyisoprenylated methylated protein methyl esterase (PMPMEase), which hydrolyzes the methyl esters of polyisoprenylated proteins to form the original proteins with free —COO− groups.
PPMTase and PMPMEase counterbalance the effects of each other. It is conceivable that the methylated and demethylated forms of prenylated proteins may be variously preferred for functional interactions by different protein targets, thus rendering PPMTase and PMPMEase very important moderators of polyisoprenylated protein function. Accordingly, manipulation of these enzymes should render significant effects on many cellular functions.
PMPMEase, through its possible regulation of the functions of various types of polyisoprenylated proteins, may exert profound effects on various intracellular events and consequently on animal physiology. Proteins such as the G-gamma subunits of heterotrimeric G-proteins of the G-protein coupled receptors, nuclear lamins, and guanine nucleotide-binding proteins such as Ras are polyisoprenylated and undergo methylation. These proteins mediate processes ranging from neurotransmitter signaling, cytoskeletal and intracellular transportation functions, cell proliferation, differentiation, and apoptosis. It could be inferred from this that aberrant levels of PMPMEase activity would be expressed through disease states such as cancers, neurodegenerative, and neuropsychiatric disorders. In fact, hyperactivity of monomeric G-proteins is implicated in an estimated 30% of cancers. Ghobrial IM, et al., Hematol. Oncol. Clin North Am. 2002, 16(5):1065-1088.
PMPMEase is inhibited by millimolar concentrations of the anticancer drugs tamoxifen and cyclophosphamide as well as by micromolar concentrations of the chemopreventive compound curcumin and polyunsaturated fatty acids such as arachidonic acid (AA). Prostaglandin (PG) A2 was 63-fold less potent than AA while PGE2 did not inhibit PMPMEase at 1 mM. AA's effects on cell death coupled with the expression of COX-2 in various tumors and tumor cell lines may imply that COX-2 converts AA into PGs, thereby destroying AA's ability to effectively inhibit PMPMEase and regulate cell growth. These results also show that balanced PMPMEase activity may be critical for normal cell viability.
It is an object of the invention to provide a method for detecting cancer, particularly breast cancer. The method involves measuring the activity of the enzyme PMPMEase.